1. Field of the Invention
The present invention relates generally to the control of output devices such as printers. In particular, the present invention relates to a circuit for improving laser printing of edges and gray shades.
2. Description of Related Art
Printers have long been used with computers to provide a written record of the data contained in the computer and memory devices. Laser printers are well known for producing quality printing at high speeds. Laser printers use a laser or diode that is selectively switched on and off to create an image of toner for deposition onto a sheet of paper. The switching of the laser is controlled by circuits that provide signals to switch the laser on and off as it scans line by line across the image to be deposited on paper. Heat is then applied to bond the toner to the paper.
Each printed image is composed of a thousands of pixels or dots very small in size. The size of the pixels is dependent on the resolution of the printer. Lasers with a resolution of 300 dots per inch (dpi) are known. Laser printers group pixels together to form various characters, numbers and graphical images.
One problem with prior art laser printers is their inability to produce shades of gray. One common method used by the prior art to produce shades of gray is dithering. Dithering produces shades of gray by grouping dots together, and printing various numbers of dots in the group to produce a single pixel with the shade of gray desired. For example, for sixteen shades of gray, the dots are group in sets of sixteen with four dots vertically and four dots horizontally. For a very light shade of gray only one pixel in the block of sixteen will be printed. For a very dark shade of gray fifteen pixels in the block of sixteen with be printed. Intermediate shades of gray are achieved by printing between 2 and 14 pixels in the block. While the dithering technique of the prior art is able to produce shades of gray, the resulting print has a very grainy appearance. Additionally, there is a significant loss in resolution by using the dithering method. For example, if the printer provides a resolution of 300 dpi, printing 16 shades of gray reduces the resolution to 75 dpi since four dots in both the vertical and horizontal direction are required to produce each pixel with gray shading.
The prior art has also attempted to produce shades of gray on laser printers with other methods. For example, attempts have been made to increase the dots per inch or control the laser directly. However, these approaches are also problematic because they require modifications to portions of the hardware such as the control board and laser. Additionally, the printing has a streaky appearance, and the laser is often difficult to control.
Another problem with the laser printers of the prior art is their inability to produce smooth diagonal lines. As noted above, all the images produced by laser printers are comprised of dots or pixels. Each pixel is substantially square in shape because they are a singe dot or a square area of dots. Thus, when diagonal lines are printed, they tend to have a step like appearance. For example, with 300 dpi printing there are visible steps when printing lines that are not strictly horizontal or vertical. The step like appearance is due to the finite size of the minimum pixel size width of the printer and the raster method often used to print pixels. The inability to produce diagonal lines also affects the printing of characters and symbols. Thus, there is a need to improve the printing of diagonal edges by laser printers.
Therefore, there is a need for an laser printer with improved edge resolution and an ability to print shades of gray.